The present invention relates to a mobile communication system with antennas for sending and receiving a plurality of antenna beams and mobile terminals moving in a fixed direction substantially. In particular, the present invention relates to ITS (Intelligent Transportation System).
A portable telephone system is one of mobile communication systems with vehicle-mounted telephone terminals. Many cells constitute the service area of this system. Each cell has generally a one-kilometer or more radius, and the deployment of the frequency is repeated for every one-kilometer.
A cordless telephone system typically such as PHS (Personal Handy-phone System) is another mobile communication system. Each cell of this system has in general a radius between several tens of meters and several hundreds of meters, and the deployment of the frequency is repeated for every length shorter than one-kilometer.
However, exchanging units used in these conventional mobile communication systems independently control the switching for every mobile terminal. Therefore, the number of the switching executed in each exchanging unit increases when the number of the mobile terminals in a cell increases. Furthermore, in the conventional mobile communication systems, if a cell size is small and also the mobile terminal moves in high-speed, the handovers may be frequently repeated and thus the PHS for example may fail to catch the mobile terminal.
It is therefore an object of the present invention to provide a mobile communication system, whereby even if the number of mobile terminals increases, the number of times of switching in an exchanging unit does not increase, under an environment in which mobile terminals move in a fixed direction substantially, for example on a road for vehicles.
According to the present invention, a mobile communication system includes a plurality of antennas for sending and receiving beams, a plurality of mobile terminals moving in a fixed direction substantially, and a zone-shift control unit for treating the moving mobile terminals locating within a predetermined area as a single moving zone and for controlling the antennas and directions of the beams so that the beams always cover the moving zone.
The present invention utilizes moving characteristics of vehicles on a road. Since the switching occurs only per zone, the number of times of the switching in an exchanging unit can be reduced. Handover is performed based on the difference between a speed of the zone and a speed of the mobile terminal, and thus the number of times of the handover can be reduced.
It is preferred that the zone-shift control unit operates to cover the zone by using a plurality of beams containing beams of neighboring antennas. Thus, the smooth movement of the zone becomes possible.
It is preferred that each of the antennas is a movable beam antenna that can smoothly sweep a coverage area of its beam. This is ideal. In the case that such antenna is used, it is also preferred that the zone-shift control unit includes a unit for covering the zone by using a plurality of beam patterns containing beam patterns of neighboring antennas and for switching the beam patterns in accordance with movement of the zone. Each of the beam A patterns for each antenna is formed by combining a plurality of beams. Furthermore, it is preferred that the antennas include at least one antenna that can modify a coverage width of its beam. Thereby, the number of mobile terminals covered by one beam is controllable.
It is preferred that each of the antennas is a fixed beam antenna that cannot move a coverage area of its beam, and that the zone-shift control unit includes a unit for covering the zone by using a plurality of beam patterns containing beam patterns of neighboring antennas and for switching the beam patterns in accordance with movement of the zone, each of the beam patterns for each antenna being formed by combining a plurality of beams. In this case, it is preferred that the zone-shift control unit includes a unit for switching the beam patterns in accordance with traffic distribution of the mobile terminals.
It is preferred that each zone uses a frequency band, and that the zone-shift control unit includes a unit for assigning at least two of the zones to an optional area in an environment. Thereby, it can prevent that the mobile terminal is not covered by a beam when the beam patterns are switched or when the movable beam reached to the end edge of the movable area is returned.
It is preferred that the zone-shift control unit includes a unit for cyclically assigning the zone moved to an outlet of an environment to an inlet of the environment. Since the zone is always controlled even when no mobile terminal exists in the zone, the control will be made easy.
It is preferred that the zone-shift control unit includes a unit for registering the mobile terminal newly entered at an inlet of an environment with an already existing zone or with a newly generated zone depending upon predetermined conditions. The predetermined conditions are that the mobile terminal locates within a coverage area of the already existing zone or that the mobile terminal locates at a position separated from the already existing zone by a fixed distance or less, and that the number of mobile terminals registered in the already existing zone does not exceed a limited number. The zone-shift control unit includes a unit for registering the mobile terminal with the already existing zone if the predetermined conditions are satisfied. Thereby, no zone is generated uselessly. Thus, no unnecessary beam is emitted from the antennas causing interference waves to reduce from occurring.
It is preferred that the zone-shift control unit includes a unit for merging two zones into one zone and for registering the merged zone depending upon first predetermined conditions when one of the two zones approaches the other zone. The first predetermined conditions are that a distance between the two zones is equal to or shorter than a predetermined distance and that a difference between the moving speeds of the two zones is equal to or lower than a predetermined speed. The zone-shift control unit includes a unit for merging the two zones into one zone and for registering the merged zone if the first predetermined conditions are satisfied. Thereby, the zone-shift control is effective not only when all zones move at the same speed, but also when zones move at different speeds, respectively.
It is preferred that the zone-shift control unit includes unit for dividing one zone into a plurality of zones depending upon second predetermined conditions. The second predetermined conditions are that a distance between the mobile terminals is equal to or longer than a specific distance or that the number of mobile terminals registered in the one zone will exceed a limited number. The zone-shift control unit includes a unit for dividing the one zone into a plurality of zones if the second predetermined conditions are satisfied. Thereby, proper zone control can be expected when the road becomes vacant from the congested state. That is, the zone is narrowed at the congested part of the road, whereas enlarged at the vacant part. Thus, the number of mobile terminals in a zone can be controlled so as to avoid the congested status in the zone.
It is preferred that the zone-shift control unit includes a unit for performing handover for a mobile terminal when the mobile terminal belonged to a first zone moves at higher speed than that of the first zone and enters into a second zone. Thus, the conventional handover control method can be used as it is.
It is preferred that each of the zones corresponds to a cell, and that the zone-shift control unit includes a unit used in a cellular telephone or cordless telephone system. Thereby, the present invention is easily applicable to the conventional system.